Memory devices commonly use high-voltage (HV) devices within a source gate to connect a bit line and source line in a NAND string to allow operations to be performed on the NAND string (pillar or array). For example, the source gate may connect the bit line and the source line to erase the NAND string. Some memory devices require relatively high voltages for specific memory operations. For example, flash memory devices can require voltages greater than 12 volts (e.g., up to 30 volts or more) for specific memory operations, such program or erasure of memory cells. Some operations, such as an erase operation, require a high voltage to be applied to the source line and the bit line to bias the selected NAND string. In an erase operation, gate-induced drain leakage can be used to deplete the NAND cells of charge by quantum tunneling causing the cell to be erased. To handle the high voltage, HV devices can be used.
Both low-voltage (LV) and high-voltage devices (e.g., transistors, other semiconductor components, etc.) typically include one or more dielectric layers, such as a silicon dioxide layer or one or more other dielectric or oxide layers. Devices are commonly rated for a limited voltage that may be safely applied across the dielectric layer, with estimated time-to-failure increasing exponentially with the voltage applied across the dielectric layer. Accordingly, the voltage applied across the dielectric layer should be managed to avoid damaging the device and to ensure industry standard reliability specifications.
Low-voltage devices are rated for relatively small voltages. Existing low-voltage devices commonly have voltage thresholds of 4 volts or less. In certain examples, the term “low voltage” is relative to voltage thresholds of similar, higher-voltage devices. Existing high-voltage devices commonly have voltage thresholds substantially greater than 4 volts, such as 30 volts or more. As technology advances, such terms will evolve.
Dielectric layers (e.g., a silicon dioxide layer) in a low-voltage device are thinner than corresponding layers in a similar, high-voltage device. The relatively thinner dielectric layer of low-voltage devices has a smaller voltage threshold that may be safely applied across the dielectric layer. Voltages above this threshold applied across the dielectric layer may damage such layers or devices and may decrease the time-to-failure for the device. Accordingly, for safe operation, voltages across the dielectric layer should be less than the threshold for low-voltage devices.
High-voltage devices have thicker dielectric layers, in contrast to low-voltage devices, which allow high-voltage devices to operate with higher voltages applied across the dielectric layer. For example, high-voltage devices may safely operate at 30 volts. High-voltage devices tend to be larger, less conductive, and slower than low-voltage devices. Accordingly, low-voltage devices may be preferred for performance of analog, digital, and mixed signal applications. Low-voltage devices, however, are not typically used within high-voltage devices, such as charge pumps or high-voltage bias circuits, due to the strict control requirements of voltages over the dielectric layer of the low-voltage device to ensure safe operation. Using low-voltage devices for some components in analog and/or mixed signal applications, such as charge pumps or bias circuits, would provide performance and/or efficiency benefits in such applications.
In NAND design, circuits may be located below the NAND strings in what is commonly referred as CMOS Under Array (CuA). Analog, digital, and sensing/muxing of bit line circuitry can be located in this area. Decreasing the size of the sensing/muxing circuitry increases the area available for other circuitry. HV devices can be 5× to 10× larger in size compared to LV devices of similar transconductance. Accordingly, replacing HV devices with LV devices below the NAND strings can create additional area that can be used for additional circuitry. The high voltage needed for operations, however, presents various challenges to safely using LV devices.